The present invention relates to improved microcapsules, and more specifically, to microcapsules that are highly impermeable and, when used in carbonless copying systems, result in greatly reduced yellowing.
Microcapsules generally comprise a core of fill material, typically having a size in the range of microns, surrounded by a wall or shell of polymeric material. The fill material may be either gaseous, liquid or solid and may be composed of a single substance, a solution or a mixture. The wall surrounding the core of fill material typically functions to isolate the fill material from an external environment until it is desired to have the fill material available, at which time the capsule wall is ruptured or removed in some fashion such as by mechanical pressure, dissolution or the like. Certain types of microcapsules are intended to remain intact without removal or rupture of the capsule wall such as microspheres, microbeads and the like.
There are many different uses for microcapsules and many different materials that may be microencapsulated. Of particular importance are the use of microcapsules in medicinal preparations, fertilizers, flavorings, detergents, laundry brighteners, perfumes, disinfectants, deodorizers, adhesives, carbonless copying systems and the like.
Numerous methods or techniques for preparing microcapsules and microcapsular products have been disclosed. A majority of these methods generally comprise providing a dispersed phase of minute discrete droplets containing the intended fill material in a continuous phase and then forming encapsulating walls or shells around the minute droplets. Specific methods for forming the encapsulating walls around the droplets include simple and complex coacervation, interfacial polymerization, polycondensation and the like. Specific patents relating to microencapsulation methods include U.S. Pat. No. 3,429,827 to Ruus, relating to an interfacial polycondensation type method, U.S. Pat. No. 3,578,605 to Baxter, relating to an acid chloride-gelatin-gum arabic dual-wall type method and U.S. Pat. No. 3,016,308 to Macaulay, relating to a urea-formaldehyde condensation type method, among others.
While microcapsules and microencapsulation techniques are applicable to a wide variety of products, perhaps one of the most significant applications is in their use in carbonless copying systems. While the present invention is particularly adaptable to carbonless copying systems and will be discussed primarily hereinafter with regard to such systems, it should be understood that the invention is not thereby so limited and may be used in any application where the improved properties of the microcapsules are beneficial.
Standard carbonless copying systems include a plurality of substrates, e.g., paper sheets, arranged in a manifold, each sheet having one or more coatings on a surface thereof. The manifold is designed so that when external pressure caused by a typewriter, pen, or other instrument is applied to the outermost sheet, a colored image will be formed on at least one surface of each sheet of the manifold.
The top sheet of the manifold to which the pressure is applied has a coating on its back surface. This coated back surface includes microcapsules containing an initially colorless chemically reactive color-forming dye precursor as the fill material. The front surface of the next sheet, which is adjacent to the back surface of the top sheet, is coated with a material containing a component, such as phenolic resin or reactive clay, that is capable of reacting with the colorless dye precursor contained in the microcapsules to produce a color. Thus, an external pressure on the front surface of the top sheet will rupture the microcapsules on the back surface and release the colorless dye precursor which then chemically reacts with the reactive component of the coated front of the adjacent sheet to produce a colored image corresponding to the area of pressure. Similarly, colored images are produced on each successive sheet of the manifold by the external pressure rupturing the microcapsules carried on the bottom surface of each sheet.
The sheets of the carbonless copying system manifold are designated in the art by the terms CB for "coated back", CFB for "coated front and back," and CF for "coated front." The CB or transfer sheet is usually the top sheet of the manifold and the sheet to which the external pressure is applied. The CFB sheets are the intermediate sheets of the manifold, each of which is able to have an image formed on its front surface by a pressure, and each of which also transmits the contents of ruptured microcapsules from its back surface to the front surface of the next sheet. The CF or recording sheet is the bottom sheet and is coated only on its front surface so that an image can be formed on it.
While it is customary to have the coating containing the microcapsules on the back surface of the sheets and to have the coating containing the reactive component for the capsules on the front surface of each of the sheets, the reverse arrangement is also possible. In addition, one of the reactive ingredients may be carried in the sheets themselves, rather than applied as surface coatings. Furthermore, the component that reacts with the colorless dye precursor may also be microencapsulated.
As indicated above, a variety of arrangements exist for providing carbonless copying systems, the most common arrangement and the arrangement to which the present invention is particularly adaptable, is a system where microcapsules having a fill material including an initially colorless, chemically reactive, color forming dye precursor are coated on the back surfaces of each of the sheets of the manifold set and a dry coating containing a reactive component for the dye precursor is coated on the front surfaces of each of the sheets of the manifold set.
The most common microcapsules used in carbonless copying systems are those composed of polyamide, epoxy, or polyurea. One disadvantage of carbonless copying systems using polyamide microcapsule systems is the inadvertent and unintentional development of color on the CF coatings and the CB sheet. This inadvertent color development may be caused by the presence of free colorless dye precursor in the CB coatings due to incomplete encapsulation of the dye precursor or may be caused by accidental microcapsule rupture which can occur during handling, coating processes, printing processes and the like. In addition, the walls of the microcapsules may include faults or pores which allow the colorless dye precursor to leak from the microcapsules. This free dye precursor often causes discoloration by contacting the CF component by passing through the base paper in the CFB sheets and also from sheet to sheet in a manifold set. Discoloration, which is variously referred to as blush, offset, bluing, ghosting, back print, etc. is highly objectionable and undesirable in a copying system.
Another shortcoming of polyamide microcapsules for use in carbonless copying systems is a tendency for the microcapsules to cause "yellowing" of the base paper, particularly when used with groundwood base papers. Such yellowing is an undesirable by-product of prior art polyamide microcapsule systems.
A further disadvantage of carbonless copying systems using polyamide microcapsules is the presence of excess amine on the surface of the microcapsules and in the microcapsule slurry. Such reactive amines are present during the production process and also in the finished product carbonless paper. These amines, such as diethylenetriamine, can be easily absorbed through human skin and can cause dermatological reactions upon contact, thus creating potential health and safety hazards both for workers and for users of carbonless paper.
U.S. Pat. No. 4,209,188 to Chao et al. discloses a technique for producing microcapsules having reduced discoloration from leakage or premature rupture. These microcapsules are produced by reacting a methylolated epoxy compound with the material of the outermost surface of the microcapsule to close discontinuities in the encapsulating wall. While the microcapsules and method disclosed by this patent address the first shortcoming discussed above, i.e., discoloration caused by premature microcapsule rupture or leakage, they do not address the problems of microcapsule "yellowing" or human exposure to amine compounds.
The principle advantage of the present invention is to provide microcapsules and a method for making microcapsules which substantially obviate one or more of the limitations and disadvantages of the described prior microcapsules.
The present invention is directed to polyamide microcapsules usable in carbonless copying systems that have highly impermeable capsule walls.
The present invention is also directed to polyamide microcapsules usable in carbonless copying systems that do not cause significant discoloration of the base paper.
The present invention is further directed to polyamide microcapsules usable in carbonless copying systems that are free of faults or pores that may allow premature rupture or leakage of the core material.
The present invention is also directed to polyamide microcapsules usable in carbonless copying systems that exhibit reduced yellowing of the base paper.
The present invention is further directed to polyamide microcapsules usable in carbonless copying systems using groundwood base papers without significant yellowing.
The present invention is also directed to polyamide microcapsules usable in carbonless copying systems that have a reduced amount of free amines to minimize hazardous exposure to humans.
The present invention is also directed to a process for producing polyamide microcapsules usable in carbonless copying systems that are highly impermeable and exhibit reduced yellowing.
Additional features and advantages of the invention will be set forth in the written description which follows, and in part will be apparent from the written description or may be learned from the practice of the invention. The advantages of the invention will be realized and attained by the microcapsules and process for manufacturing microcapsules, particularly pointed out in the written description and claims hereof.